Add to_ruby convention for custom tag compilation

Tags can now implement to_ruby(code, compiler) to provide their own
optimized Ruby code generation. This allows:

1. Third-party tags to participate in compilation
2. Shopify-specific tags to be lowered to Ruby
3. Better performance for frequently-used custom tags

Priority order for tag compilation:
1. tag.to_ruby(code, compiler) - Custom implementation
2. Built-in compiler (IfCompiler, ForCompiler, etc.)
3. Yield to caller as external tag

Also updated PROJECT.md with:
- External calls yielding documentation
- to_ruby convention documentation

lib/liquid/compile/ruby_compiler.rb

@@ -317,11 +317,14 @@ module Liquid
       end
 
       def compile_tag(tag, code)
-        compiler_class = find_tag_compiler(tag)
+        # First, check if the tag implements to_ruby (custom compilation)
-        if compiler_class
+        if tag.respond_to?(:to_ruby)
+          tag.to_ruby(code, self)
+        # Then check for a built-in compiler class
+        elsif (compiler_class = find_tag_compiler(tag))
           compiler_class.compile(tag, self, code)
         else
-          # Unknown tag - delegate to the original tag's render method at runtime
+          # Unknown tag - yield to caller at runtime
           compile_external_tag(tag, code)
         end
       end